Ozone is widely used in various fields, such as water treatment, foods, medical hygiene, semiconductor production, etc., as a strong oxidizing agent. Ozone is usually produced by a discharging method or an electrolytic method but recently an electrolytic method capable of producing ozone in high concentration has been mainly used.
The electrolytic ozone generating method is the anodic electrolytic reaction according to the formula (1) below with the oxygen generating reaction according to the formula (2) below occurring as a side reaction. EQU 3H.sub.2 O.fwdarw.O.sub.3 +6H.sup.+ +6e (1) EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e (2)
On the other hand, the cathodic reaction is usually a hydrogen generating reaction according to the formula (3) below but by supplying an oxygen-containing gas, the oxygen reduction reaction of formula (4) can also occur. EQU 2H.sup.+ +2e.fwdarw.H.sub.2 ( 3) EQU O.sub.2 +4H.sup.+ +4e.fwdarw.2H.sub.2 O (4)
U.S. Pat. No. 4,416,747 discloses a process for generating hydrogen at the cathode side and ozone and oxygen at the anode side by carrying out an electrolysis using an ion-exchange membrane as the electrolyte. Also, U.S. Pat. No. 4,541,989 discloses a process for producing ozone without generation of hydrogen from a cathode using a tetrafluoroborate as the electrolyte, an oxygen reduction cathode as the cathode, and glassy carbon as the anode. Furthermore, JP-B-2-44908 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process for generating ozone by combining an ion-exchange membrane as the electrolyte and an oxygen reduction electrode.
However, in the process described in U.S. Pat. No. 4,416,747 described above, the hydrogen generated at the cathode is originally unnecessary and a means for treating hydrogen rather becomes necessary. In addition, there is a further disadvantage in that some of the hydrogen intermixes with an anodic gas containing oxygen and ozone through the membrane with the system becoming hazardous if the explosion limit is reached. Also, the intermixing of hydrogen is not only dangerous but also in the case that ozone is generated for washing semiconductors, a disadvantage occurs because the concentration of hydrogen in the oxygen is over several hundreds ppm. This is over the upper concentration limit of hydrogen, with the result that the ozone thus generated cannot be used for washing semiconductors.
Furthermore, the theoretical decomposition voltage for generating ozone by the combination of formula (1) and formula (2) described above is 1.5 V, which is higher than the decomposition voltage of 1.2 V using conventional water electrolysis by the combination of formula (2) and formula (3), the cell voltage under a practical electrolytic conditions, for example, 100 A/dm.sup.2, is high as about 3.3 V, and hence the amount of electric power consumed becomes very high.
Also, in the process described in U.S. Pat. No. 4,541,989 described above, to obtain ozone with good efficiency, it becomes necessary to maintain the electrolytic cells at a low temperature by cooling the electrolytic cells. However, although the working characteristics of the gas electrode used for oxygen reduction are excellent at high temperature, the working characteristics thereof are quite low at low temperatures. Thus, there is the disadvantage that the process can be operated from a practical standpoint at a low electric current density only.
Furthermore, the process described in JP-B-2-44908 described above can be safely used to produce ozone at a high concentration but there is the disadvantage that the electric current density must also be kept at a low density from the standpoint of the working characteristics and hence ozone cannot be produced with good efficiency.
For generating ozone with good efficiency, it is desirable to conduct the electrolysis at a relatively low temperature of not higher than 60.degree. C. and at a current density of at least 50 A/dm.sup.2 but hitherto, a gas electrode which sufficiently functions under these conditions has not yet been found.